Xiao Li took photos of Xiao Ming when he participated in the photography activities organized by the science interest group, and found Xiao Ming taking photos.

A, the mobile phone is far away from classmates, the object distance increases, the image distance decreases, and the image becomes smaller, which conforms to the meaning of the question.

B, the mobile phone is close to the classmate, the object distance is reduced, and the image is increased.

C. The students have occupied the display screen of the mobile phone, and the mobile phone moves to the left appropriately, so the students on the right cannot appear on the screen.

D. The students have occupied the display screen of the mobile phone, and the mobile phone moves to the right appropriately, so the students on the left cannot appear on the screen.

So choose a.